Fuel cells produce electricity by converting reactants such as hydrogen and oxygen into products such as water. A fuel cell comprises a negative electrode, called a cathode; a positive electrode, called an anode; and an electrolyte situated between the two electrodes. During operation a voltage is produced between the anode and the cathode.
One fuel cell system having a potential for great practical importance uses an anode containing platinum, polymer electrolytes, and fuels derived from liquid hydrocarbons. A partial oxidation reaction chemically transforms the hydrocarbons into the desired reactant, hydrogen, and into undesirable carbon monoxide and nitrogen byproducts. The hydrogen ions present at the anode travel across a polymer electrolyte to the cathode. Upon reaching the cathode, the hydrogen ions react with oxygen present at the cathode and electrons from the external circuit to produce water and an external electric current produced by the voltage difference between the anode and cathode.
The liquid hydrocarbon fueled fuel cell scheme is a promising power source for electric vehicles because its fuels are readily available, inexpensive, and easily transported. This scheme requires no special provisions for on-board storage of the liquid hydrocarbon fuels beyond those already present on vehicles using these fuels to power internal combustion engines. Additionally, the existing motor fuel refining, storage, and delivery infrastructure already provides a supply of these fuels for transportation purposes.
Increasing the voltage between the anode and cathode is one way of enhancing a fuel cell's performance. Such a voltage increase can be obtained when the fuel cell electrodes are formed from catalytic materials. However, when catalytic poisons such as CO are present in the fuel, the anode to cathode voltage decreases. This in turn undesirably reduces the current flowing in the external circuit.
Hydrogen-oxygen fuel cells having platinum-containing catalytic anodes exhibit a measurable decrease in fuel cell voltage in cases where CO levels exceed about 1 to 5 ppm in the hydrogen fuel. It is believed that this decrease is caused by the additional electric potential needed at the anode to oxidize the carbon monoxide into carbon dioxide. This decrease in fuel cell voltage is frequently referred to as an activation overpotential.
As electric current is made available to the external circuit, the overpotential increases, and consequently decreases the fuel cell's effectiveness as a generator of electric energy.
Methods for reducing the effect of CO poisoning of fuel cell electrodes are known in the art. Some methods concentrate on processing the hydrogen fuel so as to remove as much CO as possible. Sometimes fuel treatment methods are combined in order to achieve greater effectiveness. One fuel processing method is called a water gas shift reaction, which reacts a mixture of CO and hydrogen with steam to reduce the fuel's CO concentration; unfortunately, equilibrium constraints limit conversion in the bulky water gas shift reactors so that at best the hydrogen fuel still contains between about 0.5 and 1% CO. Another method called preferential partial oxidation selectively oxidizes CO in the presence of hydrogen and can reduce the fuel's CO content, but the preferential oxidation scheme also has serious difficulties. For example, oxygen must be added during the preferential oxidation reaction resulting in the undesirable oxidation of hydrogen fuel into water. Even when preferential oxidation and water gas shift are used in combination under transient conditions, those processes result in a hydrogen fuel containing excessive CO impurities.
Other methods for reducing the effect of CO impurities on fuel cell voltage use CO-tolerant fuel cell electrodes. The amount of activation overpotential that develops at an electrode in the presence of CO impurities depends on the electrode potential that the anode requires to oxidize the adsorbed carbon monoxide. Changing the composition, electronic structure, and physical structure of the anode material can affect the amount of electrode potential required to oxidize the carbon monoxide.
Both Pt/Ru and Pt/Sn electrodes are known to exhibit CO oxidation activity at potentials lower than those observed with pure platinum electrodes. However, it is believed that electrodes made from these materials cannot tolerate CO concentrations in the hydrogen fuel in excess of about 10 ppm without exhibiting CO activation polarization. This CO tolerance is less than that needed for practical fuel cell use. Further, the observed CO activation polarization results in a 200 to 500 mV reduction in fuel cell voltage in a cell made with electrodes fabricated using these materials, thereby reducing the cell's effectiveness as an electric power generator.
Platinum particles dispersed in non-stoichiometric hydrogen tungsten bronzes are candidate electrodes for use in fuel cells. See for example, U.S. Pat. No. 5,470,673, where the electrocatalytic capability of platinum-dispersed, nonstoichiometric hydrogen tungsten bronzes and their use as fuel cell electrodes in carbon monoxide based fuel cells is discussed. However, the platinum-dispersed, non-stoichiometric hydrogen tungsten bronzes of that reference are prepared by co-electrodeposition or co-deposition processes that do not directly control the dispersion of platinum. Ideally, 50% or more of the platinum should be in the form of surface platinum. This requires platinum dispersion resulting in platinum crystals having a diameter ranging from about 20 .ANG. to about 30 .ANG.; whereas co-electrodeposition according to the method of U.S. Pat. No. 5,470,673 results in platinum crystals with a diameter of about 40 .ANG.. See Shen, et al. in J. Electrochem. Soc., 142, 3082-3090, 1994.
Platinum dispersed, non-stoichiometric hydrogen tungsten bronzes can also be prepared using simultaneous electrochemical techniques. See for example, Kulesza et al. in J. Electrochem. Soc., 136, 707-713, 1989. That these methods also result in insufficient platinum dispersion is shown by the reported platinum particle diameters ranging from about 1000 .ANG. to about 2000 .ANG..
An additional disadvantage attendant to both co-electrodeposition and simultaneous electrochemical deposition is the use of unstable precursors in forming platinum-dispersed, non-stoichiometric hydrogen tungsten bronzes.
Freeze drying is another method for preparing platinum-dispersed, non-stoichiometric hydrogen tungsten bronzes. See for example, Chen, et al. in J. Electrochem Soc., 142,1185-187, 1995. It is known, however, that freeze drying does not produce uniformly small platinum crystals.
The formation of platinum-dispersed, non-stoichiometric hydrogen tungsten bronze having uniformly small dispersed platinum particles can occur according to sequential preparation methods. See for example U.S. Pat. No. 5,298,343. Sequential preparation occurs when an oxidized tungsten species is deposited on a carbon-supported platinum electrode catalyst, and then in a separate step the oxidized tungsten species is reduced thereby forming a non-stoichiometric hydrogen tungsten bronze.
While sequential preparation methods are known, the reference teaches that the resulting materials are suitable for reducing O.sub.2 at the cathode of a phosphoric acid fuel cell and not for oxidizing CO or hydrogen at the anode of a polymer electrolyte fuel cell. Additionally, the support material of that reference is in the form of a tungsten oxide and not a non-stoichiometric hydrogen tungsten bronze.
Consequently there is a need for a CO-tolerant anode material formed from stable precursors having platinum particles dispersed in a non-stoichiometric hydrogen tungsten bronze that is capable of oxidizing carbon monoxide at low potential energy thereby minimizing the undesirable overpotential.